In a wireless communication system that performs transmission/reception using a microwave or millimeter wave band, a band-pass filter is commonly used to pass only a signal of a desired frequency band and eliminate a signal of an unnecessary frequency band. At that time, to obtain a large attenuation amount of a frequency band in a periphery of a passband without increasing the number of stages of a filter, a so-called cross coupled filter having a pole on an attenuation characteristic is used.
As the cross coupled filter, disclosed are, for example, an E-plane finline band-pass filter using a finline for a resonance element (PTL 1, FIG. 9) and an E-plane finline band-pass filter including an external cavity (PTL 2, FIG. 10). In addition thereto, a band-pass filter having a structure where a pair of waveguide resonators is connected with each other via a connection hole (PTL 3, FIG. 11) is disclosed.